Gas phase polymerization processes typically produce a gaseous stream exiting the gas phase reactor that contains fine solid particles, unreacted olefins, and other gaseous and particulate components. These fine solid particles are often referred to as polymer fines, and they include growing polymer particles, formed and agglomerated polymer particles, and catalyst or pre-polymer particles. In gas phase, fluidized bed polymerization processes, it is desirable to separate these solid fines from the exit gases and return them to the gas-phase reactor for further reaction.
Eductors are frequently used in the separation and return of the solid fines to the reactor. In commercial polymerization processes, for example continuous gas-phase, fluidized bed polyolefin polymerization processes, eductors may be used in combination with fines separators such as cyclones in this process. For example, the solid fines particles can be removed from the bottom of the cyclone by the suction provided by an ejector and recycled to the reactor. However, with typical eductor designs, it is common that the gas loop piping and equipment will foul with polymer build up in low-flow zones. Build-up may arise from the deposition of formed polymer particles or growing polymer particles that may continue polymerization of available olefins. The extent of this problem may be sufficiently severe that a shutdown is periodically required for cleaning, in some instances as often as every few months. Such incidents are costly from both time and economic standpoints. Eductors can provide the motive force necessary to transfer the stream comprising fine particles back into the reactor.
Various solutions to the problem of eductor fouling have been proposed. For example, the eductor may periodically be flushed with a nitrogen purge or a continuous motive gas purge, whereby the cyclone and eductor are purged with a stream of high pressure gas, to remove any collected fines and prevent further eductor fouling. However, eductor purges cause down time in the reactor and eductor operations, resulting in extremely high operating and maintenance costs. Various eductor designs have also been explored in an attempt to address this issue and improve metrics such as reactor run time from a mechanical solution standpoint.
Therefore, there is an ongoing need for improved eductor designs which might reduce the incidence of polymer fouling. There is also a need for new eductor designs which do not require frequent high pressure nitrogen or liquid purges, and which may thereby increase efficiency and profitability of the reactor systems with which they are associated.